This invention relates to optical communications devices and, more particularly, to an optical device that splits an optical beam into three separate frequency bands.
In an optical communications system, information is encoded onto a light signal. The light signal is transmitted from one point to another, as for example by free-space light beams or optical fibers. At the receiving end, the information is read from the light signal.
An important advantage of optical communications is that a number of different light signals of different frequencies may be mixed together (multiplexed) onto a single light beam in a technique known as wavelength division multiplexing (WDM). Each light signal of a different frequency (and thence wavelength), or channel, has information encoded onto it prior to the mixing of the channels. At the receiving end, the channels are separated, or demultiplexed, according to their frequencies. In one approach, a series of light bandpass filters are used to sequentially extract the channels, one at a time, from the light beam. The information on each channel is read from the demultiplexed light of that frequency. A single multiplexed light beam may therefore carry many times the information that may be transmitted by a non-multiplexed light beam.
As the number of channels in the multiplexed beam increases, the power carried by the beam increases. At the receiving end, each of the optical devices used to extract the channels must function at the power level carried by the optical beam at that point. The light bandpass filters may be made with the capability of functioning under high power levels, but providing this capability adds to their cost.
There is a need for an approach that allows the power levels on the individual channel separators to be reduced, while at the same time not degrading the quality of the optical beam or interfering with the demultiplexing function. The present invention fulfills this need, and further provides related advantages.
The present invention provides an optical frequency discriminator that divides an optical input beam into exactly three separate frequency bands. These frequency bands may each contain a number of channels of information mixed into the light of the frequency bands. The channels are thereafter demultiplexed using conventional techniques. The present approach reduces the power loading on the demultiplexing optical devices, so that they may be constructed less expensively than would otherwise be the case. The present apparatus is compact and relatively simple and inexpensive to build. It is tunable to optimize its performance. The present discussion is in terms of frequency of the light, but it is equally applicable to a discussion of wavelength.
In accordance with the invention, an optical system includes an optical frequency discriminator comprising a light source of an optical input beam, and an optical bandpass filter having a bandpass filter range and the optical input beam incident thereon at a bandpass-filter angle of incidence. The optical bandpass filter transmits a first transmitted beam of a bandpass-filter frequency band and reflects a first reflected beam. An optical second filter has an optical edge in the bandpass range of the bandpass filter and the first reflected beam incident thereon at a second-filter angle of incidence. The optical second filter transmits a second transmitted beam and reflects a second reflected beam. Desirably, the optical frequency discriminator further includes an optical bandpass filter angular adjustment that alters the bandpass-filter angle of incidence of the optical bandpass filter relative to the optical input beam, and/or an optical second filter angular adjustment that alters the second-filter angle of incidence of the second filter relative to the first reflected beam. The optical second filter may be a low-pass filter that transmits light having a frequency less than the optical edge and reflects light having a frequency greater than the optical edge, or a high-pass filter that transmits light having a frequency greater than the optical edge and reflects light having a frequency less than the optical edge. The optical system typically includes a first optical device that receives the first transmitted beam, a second optical device that receives the second transmitted beam, and a third optical device that receives the second reflected beam. The first optical device, the second optical device, and the third optical device preferably each comprises an optical demultiplexer.
Stated alternatively, an optical frequency discriminator comprises a light source of an optical input beam, and an optical bandpass filter having a bandpass filter range and the optical input beam incident thereon at a bandpass-filter angle of incidence. The optical bandpass filter transmits a first transmitted beam of a bandpass filter angle and reflects a first reflected beam including a first reflected frequency range less than the bandpass-filter range and a second reflected frequency range greater than the bandpass-filter range. An optical second filter has the first reflected beam incident thereon at an optical-second-filter angle of incidence. The optical second filter transmits one of the first reflected frequency range and the second reflected frequency range and reflects the other of the first reflected frequency range and the second reflected frequency range. The optical second filter is preferably but not necessarily an edge filter. Most preferably, the optical frequency discriminator has exactly these two optical filtersxe2x80x94the optical bandpass filter and the optical second filterxe2x80x94and no other optical filters.
The optical frequency discriminator preferably splits the optical input beam into exactly three component bands, which may then be passed to the demultiplexer devices. Because the optical input beam is split into components, the demultiplexer devices need only be operational with beams of lower power than would be the case if the full-power optical input beam were incident upon the demultiplexer devices. Thus, while the optical bandpass filter must be suitable for operation at the full beam power, it is the only filter with that requirement. The other filters may be sized and constructed for lower-power operation.
The optical frequency discriminator is tunable to optimize its performance for the specific frequencies of the optical input beam. The tuning is accomplished readily and mechanically, by changing the angle of incidence of the beams on the filters.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.